Incorrect bucket number in bucket info & document the role of `setting`

[OscarWang114]

In processors/text_classification.py, feature "label" has the following bucke info:

"label": feature.Value(
    dtype="string",
    description="category",
    is_bucket=True,
    bucket_info=feature.BucketInfo(
        method="bucket_attribute_discrete_value", number=4, setting=1
    ),
),

The number should be 2 (for label 0 and 1) not 4? Also, setting = 1 seems to correspond to the number of entities after digging the code, but we can make its role clearer. Would be nice if we can make it a unified setting data class for all types of buckets.